1. Field of the Invention
This invention relates generally to a circuit for distinguishing between speech and non-speech signals and, more particularly, to a circuit for use in a recording/reproducing apparatus that is voice controlled.
2. Description of the Prior Art
In a recording/reproducing apparatus using a magnetic tape, a solid-state memory, a magnetic disk, or the like, as a recording medium, it is known to conserve the space available on the recording medium by automatically setting a recording mode to record speech signals only when a person is actually speaking. These recorders are known as voice actuated or voice operated recorders and applications for such recording/reproducing apparatus are an automatic telephone answering machine, a memory machine, a transcription machine, and the like. In the apparatus to be voice controlled, a circuit for distinguishing between speech and non-speech signals, that is, which judges the presence/absence of an input speech signal, is typically employed.
A conventional speech/non-speech signal judging circuit compares the level of an input speech signal with a predetermined threshold level, determines that the speech signal is a non-speech signal when the speech signal is lower than the threshold level, and determines it to be a speech signal when it exceeds the threshold level.
In the conventional speech/non-speech signal judging circuit, however, the threshold level for distinguishing between speech and non-speech signals is fixed at a predetermined value. Therefore, when there is a large, steady, noise disturbance, such as unusual ambient noise picked up by a microphone or a telephone or a telephone line, even if the user does not speak, the noise level exceeds the predetermined threshold level and the presence of a speech signal is erroneously detected. As a result, the recording/reproducing apparatus is undesirably set in the recording state and this disturbance noise is erroneously recorded, thereby decreasing the utilization rate of the recording medium and defeating one of the original purposes of the voice actuated recorder.
This problem can be particularly troublesome in an automatic telephone answering apparatus wherein the telephone line is disengaged by detecting a non-speech signal, that is, the absence of speech, upon completion of a message from a caller. If a detection error is caused by noise, the telephone line will be kept DC-engaged even after the message is completed. For this reason, in addition to wasting the available space on the recording medium, the automatic telephone answering apparatus cannot prepare for the next incoming call because the telephone line has been incorrectly kept engaged.
In the Voice Operational Recording (VOR) mode of a dictating or transcription machine, the recording state can also be automatically started by a large noise disturbance, and the recording state will unnecessarily continue. As a result, an actual input speech signal may not be able to be recorded because the recording medium has been used up.